Systems used to electronically capture a picture image using an image pickup device, such as a text imaging camera, are well known. For example, a presentation system is well known that uses a text imaging camera to form an image of a written document, or similar document, as an electronic image and then displays a greatly enlarged version of this image using a device that provides a greatly enlarged image, such as a projector. Additionally, this image may be sent through a teleconferencing system that includes a communications network in order to display versions of this image at various remote locations.
With image pickup devices such as a text imaging camera, an image pickup device such is a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) is used. Imaging lenses for use with such image pickup devices have been developed for use in digital cameras. For example, Japanese Laid-Open Patent Application 2001-021800 and Japanese Laid-Open Patent Application 2001-100092 disclose imaging lenses developed mainly for use in digital cameras and that include five lens elements with some aspheric surfaces in the lens system.
However, with a presentation system that uses a text imaging camera, as described above, an original written document having characters and similar finely detailed information on it must be imaged in the text imaging camera with high optical resolution in order for a final highly enlarged projected image to be a close replica of the original written document. Additionally, because the original written document is imaged at close range and in order to be compatible with a large document, a wider picture angle, that is, a wider field angle of the imaging lens than an ordinary digital camera provides is needed. The suppression of distortion aberration with high resolution is especially difficult, but necessary, with a wide picture angle provided by a wide-angle imaging lens in a text imaging camera or similar device.
In order to satisfy the above requirements, the number of lens elements and lens components used tends to increase, including, for example seven lens elements, in order to suppress aberrations, especially distortion aberration. With this increase, the outer diameters of the lens elements, along with the total length and weight of the entire optical system, tend to increase. However, in recent years, peripheral components for mounting an imaging lens on the image pickup device side have been getting smaller, and this necessitates making the accompanying imaging lens system smaller as well.
The imaging lenses in the two Japanese applications discussed above use a comparatively small number of lens elements, only five, and the correction of aberrations is comparatively favorable, based on their use of aspheric surfaces. However, the lenses described in these Japanese applications are designed for use primarily in digital cameras and not adequately suited for a wide picture angle as needed in a text imaging camera. Furthermore, because the imaging lens of Japanese Laid-Open Patent Application 2001-100092 uses an aspheric surface in a cemented lens component, the imaging lens is expensive to manufacture, making it impractical for mass production.